The invention relates to means for rapidly stabilizing acrylic fibers by precisely controlling partial oxidation in an oxidizing atmosphere to a density level at which the fibers will not burn when subjected to an ordinary match flame and are capable of sustaining conventional carbonization temperatures to produce carbon fibers. The process of the invention involves the utilization of selected treatment temperature modes for the fiber as it passes through varying density ranges during oxidative stabilization. Acrylic fibers, as referred to throughout the specification and claims, are acrylonitrile homopolymer fibers and copolymer fibers containing at least about 80 mol % acrylonitrile. These fibers are routinely supplied in the form of tows comprising continuous multifilament bundles conventionally containing about 1,000 to about 160,000 individual fibers.
The thermal stabilization of a bundle of acrylic fibers historically has required a heat treatment of relatively long duration (e.g., elapsed time of at least about 4 hours). A lengthy heating period has normally been required to produce a density level at which an acrylic fiber bundle is non-burning when subjected to an ordinary match flame and will withstand carbonization temperatures, in view of the fact that rapid heating during stabilization to temperatures in the vicinity of the exothermic transition point of a fiber bundle produces "run-away" intermolecular cross-linkage reactions which result in local accumulation of heat. These "hot spots" in the bundle cause uneven heat distribution and result in the formation of a highly viscous liquid substance which, at the temperature of formation, causes fusion (i.e., a bonding) of the individual fibers, and may result in complete fragmentation of the fiber bundle. The fusion temperature is defined as that temperature at which formation of the highly viscous liquid substance is initially observed to form. The extensive time required for acrylic fiber stabilization has been a primary cause of relatively low production rates and associated high manufacturing costs for commercial carbon fiber production.